I have a bungalow extension about to start. Two rooms will be knocked together as part of the extension work. Checking floor levels has revealed a 50mm lower floor level that will need to be brought up to the new level. The floor area is the entry hall, small study and dining room in total about 20 square meters, and will be finished with engineered wood flooring. What is the best way of making up 50mm ? the builder has suggested a 50mm screed, but I would like to take the opportunity to add insulation.